1. Field of the Invention
This present invention relates generally to methods and apparatus for processing semiconductor wafers using plasma etch techniques. In particular, this invention relates to a system and a concomitant methodology for optical emission detection to determine the optimum endpoint of a process step such as when the etching of a film on a wafer is complete.
2. Description of the Prior Art
Plasma processing is an established part of semiconductor wafer manufacturing processes that require etching. In the semiconductor industry, plasma etching is used for removing layers from a semiconductor wafer or for etching patterns in a layer of material overlaying a semiconductor wafer. Plasma processing is used to etch precise patterns in layers of polycrystalline silicon, silicon nitride, silicon dioxide, and the like, using a patterned photoresist layer as an etch mask. Plasma processing is also used for stripping photoresist layers, descumming wafers, or other cleaning steps.
It is desirable to have a highly controlled plasma etch process, as well as an automatically controlled process. To ensure that the desired etching and deposition occurs, it is important to closely control the plasma environment throughout the entire fabrication process. The plasma pressure, the ion concentration level and the relative volume(s) of the reactant gases have been used, usually in combination, to control the plasma environment. Other parameters and physical elements such as RF power, gas mixtures and flows, reactor chamber pressure, and substrate temperature and wafer loading factors also have been used to control plasma etching processes. The interaction of these parameters with respect to the plasma gas phase chemistry is extremely complex, thus making process repeatability difficult and process optimization poor. Other control techniques have been proposed and utilized in the past, including, for example, merely setting the time for the total reaction; detecting the completion of the reaction using a photodetector; detecting the onset of etching and then allowing the reaction to continue for a set length of time. While all of these techniques provide a degree of control, they are inadequate for precise process control and for the detection of the endpoint of a process step.
Optical emission spectroscopy is now utilized more commonly to control the plasma by identifying and monitoring chemical species in the plasma. In current spectroscopy plasma monitoring systems, a series of real time spectra displays are generated, each of which serves as a snapshot of the chemical constituents of the plasma at the time the spectrum was taken. Each spectrum display takes the form of a graph which plots the intensity of light emitted within the chamber versus wavelength over a designated frequency range. The displays are typically characterized by a multiplicity of spaced apart intensity peaks, with each significant intensity peak corresponding to the presence of a specific chemical species within the plasma. Accordingly, each chemical species having a significant volume within the plasma will appear as an intensity peak at the specific wavelength emitted by that species. Once the intensity peaks are identified from a library of waveform peaks associated with a specific chemical species that are expected in a given plasma process step, the volume or mass of each identified species can be ascertained from the magnitude of the intensity peak.
During etching, the different materials and elements in the plasma emit different wavelengths of light. As etching progresses, the composition of the plasma will change as the ratio of the different elements comprising the plasma changes. An endpoint detector detects these changes as variations in the intensity of the emissions being monitored and produces a signal representing these changes. The etch process is controlled by programming the detector to detect endpoint and to end the etch step when the signal fulfills preset parameters determined by the user.
For example, fluorine radicals are the main etch species used for etching tungsten. Fluorine radicals emit light with a wavelength of 704 nanometers (nm). A detector, mounted at the plasma chamber viewport, produces signals representing emissions from the various elements present in the plasma using selectable bandpass filters. While tungsten is being etched, there is little fluorine present in the plasma since it is being consumed. Therefore, the signal representing the fluorine emission is weak. When the majority of the tungsten is etched away and the adhesion layer is exposed, the fluorine signal intensity increases dramatically since more fluorine becomes present in the plasma. Eventually, the intensity of the emissions and corresponding signal produced by the detector will level off, indicating that the tungsten has been almost completely etched away and fluorine is no longer being consumed. When this point is reached, the process step is terminated.
Thus, by detecting changes in the magnitude of the intensity peaks of selected chemical species, the completion or endpoint of the wafer fabrication process steps in a plasma may be determined. Representative of prior art in automatic control of plasma etching systems is U.S. Pat. No. 4,491,499 issued to Jerde et al (Jerde). Jerde discloses a method for determining the time at which a plasma etching operation should be terminated. The method comprises three essential steps: (1) monitoring the optical emission intensity of the plasma in a narrow band centered about a predetermined spectral line indicative of the gas phase concentration of a plasma etch reactant or product; (2) monitoring the optical emission of the plasma in a wide band centered about the spectral line indicative of the optical background emission signal; and (3) generating a remainder intensity formed as the linear combination of the spectral line intensity and the background emission signal. The etching process is terminated whenever the remainder intensity and/or its time derivative achieves a predetermined value. The teachings and suggestions of this invention merely utilize the well-known principle of essentially subtracting from a weak, information-bearing signal a measure of the wide-band noise signal corrupting the weak signal to, in effect, cancel the noise masking the weak signal. The decision to terminate the etch step still relies on only one spectral line signal which is the same one conventionally relied upon to make the etch step termination decision.
In addition, U.S. Pat. No. 4,312,732, for example, discloses a system for detecting endpoint on the basis of the intensity of the light emitted by the glow discharge. When the intensity, as represented by a variable voltage, reaches some predetermined level, the process is terminated. U.S. Pat. No. 4,246,060 discloses a system for detecting a temporary uniformity of the voltage as indicative of optimal endpoint. See also Savage, Richard N., "Applications of Optical Emission Spectroscopy to Semiconductor Processing," Spectroscopy, Vol. 2 (No. 8), pp. 40-42, (1987) and associated U.S. Pat. No. 5,014,217.
The higher the amplitude of the signal representing the element of interest in the plasma, the better for detecting, and for detecting changes in, the presence of that element during a process step. The amplitudes of such signals vary substantially from element to element. For example, in the past, a 380 nm bandpass filter has been used when etching titanium nitride (TiN) to detect the nitrogen emission as the presence of this element changes in the plasma. However, the typical signal obtained from etching TiN is weak resulting in unreliable control of such etching steps. Accordingly, there is a need for improving endpoint detection in plasma etch systems.